1. Field of the Invention
The present invention relates to a display device such as an active matrix liquid crystal display device and, more particularly, to a monolithic active matrix liquid crystal display device comprising a substrate that has driver circuits as well as an active matrix circuit.
2. Description of the Related Art
An active matrix display is shown in FIG. 2 and has scanning lines 201 and signal lines 202 meeting at intersections. A liquid crystal material 204 is disposed as a pixel at each intersection. Every pixel is equipped with a switching device 203. Information about the pixels is represented by turning on and off the switching devices. For example, a liquid crystal material is used as a display medium in such a display device. In the present invention, thin-film transistors (TFTs) each having three terminals, i.e., gate, source, and drain, are used as switching devices.
In the present specification, a row of a matrix construction means scanning lines (gate lines) which extend parallel to the row and are connected with the gate electrodes of the TFTs in the row. A column of the matrix construction means signal lines (source lines) which run parallel to the column and are connected with the source (or drain) electrodes of the TFTs in the column. A circuit for driving the scanning lines is referred to as a scanning line driver circuit. A circuit for driving the signal lines is referred to as a signal line driver circuit. A thin-film transistor is referred to as a TFT.
FIGS. 3(a) and 3(b) show one conventional active matrix liquid crystal display. This liquid crystal display uses TFTs made of amorphous silicon to build an active matrix circuit. A scanning line driver circuit and a signal line driver circuit are each made of an integrated circuit using a single-crystal silicon substrate. Typically, a single-crystal silicon driver circuit IC 301 is mounted to the outer periphery of an active matrix circuit 302 made up of amorphous silicon TFTs by TAB techniques (FIG. 3(a)). Alternatively, a single-crystal silicon driver circuit IC chip 303 is mounted by COG (chip-on-glass) techniques (FIG. 3(b)).
This conventional liquid crystal display suffers from the following problems. First, the reliability poses problems, because the scanning and signal line driver circuits are connected with the scanning lines and the signal lines, respectively, of the active matrix circuit by TAB or wire bonding.
For example, where the display device is built in accordance with the video graphics array (VGA) standards, there exist 1920 signal lines and 480 scanning lines. As the resolution is enhanced every year, their numbers tend to be increased. Where a viewfinder for use in a video camera or a projector using a liquid crystal display is manufactured, it is necessary to make the whole display device compact. A liquid crystal display manufactured using TAB is unsuited for such applications because this display needs a large space.
TFTs built, using polysilicon, have been developed to fabricate an active matrix liquid crystal display free of the foregoing problem. One example is shown in FIG. 4, where an insulating substrate 400 is made of glass or the like. On this substrate 400, a signal line driver circuit 401 and a scanning line driver circuit 402 are constructed from polysilicon TFTs simultaneously with TFTs forming an active matrix circuit 403. Polysilicon TFTs can be fabricated by high-temperature polysilicon processes. That is, high-temperature polysilicon TFTs are formed on a quartz substrate at a temperature higher than 1000.degree. C. Also, low-temperature polysilicon TFTs can be manufactured on a glass substrate by low-temperature processes at temperatures below 650.degree. C.
The mobilities of amorphous silicon TFTs are approximately 0.5 cm.sup.2 /V sec. On the other hand, the mobilities of polysilicon TFTs can be set 30 cm.sup.2 /V or higher sec and thus they can be operated with signals having frequencies only on the order of MHz.
Both digital and analog driver circuits are available to drive active matrix liquid crystal displays. Since a circuit of digital construction has much more devices than a circuit of analog construction, it is customary to use an analog driver circuit where polysilicon TFTs are employed. In one type of scanning line driver circuit and signal line driver circuit, shift registers are utilized. In another type, decoders are used. Driver circuits using low-temperature polysilicon TFTs as described above have the following disadvantages.
The low-temperature polysilicon TFTs process have smaller mobilities and larger threshold values than single-crystal silicon transistors. Therefore, assuming that the TFTs are driven at more than 1 MHz to sample the input video signal, it is necessary to set the power-supply voltage to about 15-18 V, for example.
However, a circuit for controlling a driver circuit of a liquid crystal display is normally made of an integrated circuit using single-crystal silicon. The operating voltage is approximately 5 V. Therefore, the output signal is also approximately 5 V. Under this condition, it is difficult to control the driver circuit consisting of low-temperature polysilicon TFTs.
In recent years, ICs made of single-crystal silicon tend to be driven by decreasing power-supply voltages. If the power-supply voltage for a control circuit is 5 V, a level-shifting circuit operating digitally and consisting of N-channel TFTs 501, 502 and P-channel TFTs 503, 504 can be constructed, as shown in FIG. 5, by suppressing the threshold value to about 2 V. This level-shifting circuit is employed in an ordinary CMOS circuit at low frequencies. However, where the power-supply voltage for the IC is 3 V, it is difficult to operate the above-described level-shifting circuit even at decreased frequencies unless the threshold value is set less than 1 V.
Accordingly, it is common practice to insert a level-shifting circuit 604 made of a proprietary single-crystal IC or externally attached transistors between a control circuit 605 made of single-crystal silicon and each of a signal line driver circuit 601 and a scanning line driver circuit 602, as shown in FIG. 6, to drive an active matrix circuit 603 fabricated on an insulating substrate 600 made of glass or the like. The signal line driver circuit 601 and the scanning line driver circuit 602 are fabricated from low-temperature polysilicon TFTs on the substrate 600. Where these level-shifting circuits are attached externally, if more signals are treated, the number of externally attached level-shifting circuits are increased accordingly. This leads to an increase in the cost.